Cleaning and shaping root canals for receiving filling substance is accomplished using drilling instruments which have a conical active portion called the working portion that comprises several cutting edges arranged in a spiral along the working portion.
An example of this type of dental instrument is illustrated by International Publication No. WO 2007/016278 A1 which describes a generally conical endodontic instrument comprising several spiral flutes with variable spaces between the spirals according to the sectors of the active portion. These instruments, called endodontic files, are designed for manual or mechanized use. When the endodontic files are driven by an electric motor they rotate at a slow speed (for example between 150 and 600 rotations per minute) and progress through the canal opening in such a way that their active area can cut or scrape the walls of the root canal. Areas that were reduced in length at the beginning of the intervention extend farther and farther as the files advance through the canals. The drive torque that must be applied to the files in order to overcome the increasing force of friction and to make the files turn at the desired speed as they advance increases more and more. Progression through the canal further increases the risk of the file becoming blocked or tighten. When this occurs the torque applied to the file increases strongly and there is a risk of the file breaking. File breakage inside the root canal is an outcome the dentist dreads, since the broken point is generally irretrievable.
The endodontic instrument which is the object of Publication No. EP 1 752 109 comprises several, specifically three, spiral flutes defining the concave surfaces whose extremities are located on a circumscribed circle. These extremities constitute the peaks of the instrument's working angles and at least one side of these angles is constituted by the concave flute surfaces. It happens that these angles are all negative or neutral so as to form scraping angles, but never cutting angles, since only positive angles are capable of performing cutting operations. In order to be positive, the tangents to the sides of the angles must be situated on the same side of the radius of the circumscribed circle that corresponds to the peak of the angle. Therefore, this instrument cannot be used as an instrument that cuts sufficiently to form a root canal.
Furthermore, the instruments currently available on the market for implementing the new technique of using mechanically driven files are derived directly from traditional spiral shaped instruments for manual use. This spiral formation is often the source of the tightening and blocking phenomenon that can cause instrument breakage.
Another problem that may occur is that the instrument, instead of following the original shape of the root canal within the sharply curved areas, may have a tendency to carve out its own path or deform the original canal walls when the flute edges are too sharp. One solution consists of blunting the cutting edges of the instrument to a greater or lesser extent in order to prevent them from cutting the material and boring a path that does not correspond to the natural shape of the canal. This can be accomplished by forming an enlarged flat radial area on an endodontic instrument in which there is a large separation between the flute edges. This geometry allows the edges to be blunted, but it has the drawback of increasing working friction and heating the instrument, running the risk of breakage. In addition, enlarging the flat radial area makes the instrument more rigid, resulting in diminished flexibility and an increased risk of uncontrolled deviation within the curved areas of the root canals.
Finally, the walls of a root canal require different treatments along the length of the canal so that the dental surgeon must adapt the instruments according to the desired result. Current instruments do not allow variable treatment sequences to take place throughout the length of the root canal, but require the use of a series of instruments that vary in shape from one instrument to the next.